1. Field of the Invention
The present invention relates to display technology. More particularly, the present invention relates to organic electroluminescent devices.
2. Description of the Related Art
Organic luminescence or organic electroluminescence is an instance in which electric current is converted into visible light by internal processes of organic compounds. Organic compounds having either fluorescent or phosphorescent characteristics generate light emission upon the application of electric current. Both organic fluorescent and phosphorescent molecules are referred to as organic light-emitting compounds.
An organic luminescent or electroluminescent device is generally composed of two opposing electrodes and at least one layer interposed between the two electrodes. The at least one layer between the electrodes contains at least one organic light-emitting compound. Electrons and holes, collectively referred to as carriers, are injected into the layer from the two electrodes, which are a cathode and an anode. Upon application of appropriate electric power, the cathode is to inject electrons toward the interposed layer, while anode is to inject holes toward the same layer.
Performance of organic electroluminescent devices depends on, among many factors, the quantity of the carriers injected from the electrodes. The more the carriers are injected, the more likely to have high luminescence or brightness. To inject a large number of carriers from the electrodes at a low driving voltage, materials for the electrodes are chosen such that the carriers are easily injected therefrom. The anode is generally made from materials having a high work function. The cathode is generally made from materials having a low work function. In a single electroluminescent device, the work function of the anode material is higher than that of the cathode material.
Further, to inject a larger number of carriers from the electrodes at a low driving voltage, carrier-injecting layers may be introduced. The carrier-injecting layers are to facilitate injection of carriers from the electrodes. A hole-injecting layer may be formed on the side of the anode, facing the cathode. An electron-injecting layer may be formed on the side of the cathode, facing the anode. Materials for the carrier-injecting layers are also chosen such that the carrier-injecting layers easily receive carriers from the electrodes. The hole-injecting layer is generally made from materials having a low oxidation potential, which are easily oxidized at a low electric potential applied. The electron-injecting layer is generally made from materials having a low reduction potential, which are easily reduced at a low electric potential applied.